PEG as Phase-Transfer Catalyst

Phase-transfer catalysis could be a powerful and widely used technique for conducting heterogeneous reactions between two or more reactants in two or more immiscible phases, by employing a PTC to transfer one of the reacting species from one phase into a second phase where the reaction can take place [44, 45], 

PEGs have been extensively investigated as PTCs in many commercial processes to replace expensive and environmentally harmful PTCs [46, 47], Compared with the commonly used PTCs, linear PEGs are much cheaper than analogous macrocyclic crown ethers and cryptands [48]. PEGs are also more stable at high temperatures, up to 150-200 °C, and show higher stability to acidic and basic conditions than quaternary onium salts [49]. 

Traditional polar organic solvents are used in PTC to obtain a high rate of ion transfer and to increase reaction rates but less environmentally compatible. 

Non-volatile solvents eliminate the health and environmental risks associated with the use of volatile solvents, but may pose their own risks and separation problems. Several liquid polymers, such as PEG, poly(propylene glycol) (PPG), poly(tetrahydrofuran) (PTHF), Polydimethylsiloxane (PDMS), poly(methylphenyl siloxane) (PMPS), and variations of those with ether or ester end-capping groups, are compared in terms of environmental risk, solvent polarity, and performance as 

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In recent years, there have been a number of papers describing the use of solid support bonded PEGs as phase transfer catalysts. One of the major interests in binding PEG derivatives to solid supports is to facilitate the ease of catalyst... 

Davidson, R. S., Patel, A. M., and Sufdar, A., Alkylation of aromatic amines using PEG as phase transfer catalyst, J. Chem. Res., 1984, 88. 

Lanthanide ions (La3+, Ce3+) were found to promote the cobalt-catalyzed reaction in a two-phase system with p-cyclodextrin or PEG-400 (polyethylene glycol 400) as phase-transfer catalysts.134... 

Poly(ethylene glycol)s (PEG) and their dimethyl ethers have been used as phase-transfer catalysts for the reaction of aryl halides with diphenylamine. Since mixtures of triarylamines were obtained from a single aryl bromide (for example, u-bromotoluene gave both 0- and m-tolyldiphenylamine), arynes are intermediates. A kinetic study led the authors to propose the following mechanism ... 

While PEGS can themselves serve as phase-transfer catalysts [86], onium salts are generally more effective as catalysts. Using the chemistry shown in Eq. 18, a methoxy-PEG5ooo derivative 47 was first treated with the Cs salt of 4-hydroxybenzyl alcohol to form the alcohol 48. Conversion of the alcohol to the bromide followed by reaction with tributylamine produced a quaternary ammonium salt 49. This salt was as active as low molecular weight salts in typical phase-transfer catalyzed reactions like those of alkyl halides with KI, KCN, phenol, and pyrrole [88]. Yields were often in the >90% range. Reactions were typically carried out at <40 °C and could be performed either with water or without solvent. Control experiments showed that the ammonium group of 49 was necessary as the simple alcoholic PEG derivative 48 was much less effec-... 

Aliphatic hydrocarbons are immiscible wifli PEGs. Therefore, it is important to select a good organic-phase solvent such as aromatic hydrocarbons, chlorinated hydrocarbons, or acetonitrile. In toluene, PEGs are more effeetive eatalysts than crown ethers for the reaetion of benzyl chloride and soUd potassium acetate. In butanol, the effeetiveness of PEGs and crown ethers as phase-transfer catalysts were the same for die reaction of benzyl chloride and solid potassium acetate. ... 

Various alkyl p-tolyl sulfones were prepared under relatively mild conditions using PEG 400 and PEG 1000 and a,CD-diethylether of PEG 1000 as phase transfer catalysts [171]. 

Angeletti, E., Tundo, P, and Venturello, P, Ether synthesis using poly (alkylene glycol) as phase transfer catalysts,/. Chem. Soc., Perkin Trans., 1, 1137, 1982. Slaoui, S., Le Gouller, R., Pierre,/. L., and Luche,/. L., Application of PEG S as phase transfer catalysts in Williamson synthesis. Tetrahedron Lett., 23, 1681, 1982. 

Bartach, R. A., and Yang, D. W, Aryldiazonium salt reactions in the presence of PEG S as phase transfer catalysts. Tetrahedron Lett., 1979, 2503. 

Sukata, K., Synthesis of isoprene by decomposition of dioxane derivatives using peg s as phase transfer catalysts, J. Chem. Soc. Japan, 57, 613, 1984. 

In addition to these uses, PEG s, their derivatives, and the oligomeric glymes are being increasingly used as phase transfer catalysts (PTC s). In the present paper we describe the chemistry we have developed for synthesis of PEG derivatives, briefly review the subject of PTC by PEG s and related molecules, and describe our work on the use of PEG s and derivatives as soluble, recoverable PTC s. 

PEG is a liquid polymer that possesses several green properties suitable for use as an alternative solvent for organic transformations [99]. Moreover, its miscibility with water, ease of recyclability, and the ability to act as phase-transfer catalyst has prompted its use as an eco-friendly solvent. A series of imidazole-containing 1,5-benzodiazepines 145 have been prepared from condensation of appropriate chalcones and OPD (Scheme 30). Some of the prepared compounds were found to possess antibacterial and antifungal properties [100]. 

Another palladium-catalyzed coupling reaction that has been successfully performed on soluble polymers is the Sonogashira coupling. Xia and Wang have presented an approach in which the PEG 4000 utilized simultaneously serves as polymeric support, solvent, and phase-transfer catalyst (PTC) in both the coupling and... 

If, however, PEG-400 is employed as the solvent and phase transfer catalyst, under a nitrogen atmosphere, then the monoacid is obtained in good yield. Since vinylic dibromides are easily synthesized from carbonyl compounds, this constitutes a valuable method for oxidative homologation(19). 

Oxidation of alkenes to ketones.3 Both internal and terminal alkenes are oxidized by PdCl2 (with CuCl2 as reoxidant) in water-polyethylene glycol (PEG), serving as the phase-transfer catalyst as well as the solvent (9, 360, 376). This oxidation is more facile than that catalyzed by quaternary ammonium salts, which is applicable only to terminal alkenes. 

Sauvagnat, B., Lamaty F., Lazaro, R. and Martinez, J., Polyethylene glycol (PEG) as polymeric support and phase-transfer catalyst in the soluble polymer liquid phase synthesis of ct-amino esters, Tetrahedron Lett., 1998, 39,821. 

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